1. Field of the Invention
The invention relates to a vacuum booster for reducing the force required for stepping on the pedal for operating a brake system for automobiles and, more particularly, to a rubber disc type reaction force apparatus for a vacuum brake booster for automobiles.
2. Description of the Prior Art
There have been heretofore developed two types of reaction force appartuses for brake boosters. One such apparatus is of a so called lever type and the other is of a rubber disc type which is shown in U.S. Pat. No. 3,754,450, for example. The latter type is considered simpler in construction and easier to assemble as compared with the former type.
The prior art disclosed in U.S. Pat. No. 3,745,450 discloses a rubber disc type disposed between an output rod and a power piston. In this prior art reference, the output rod is not directly contacted with the power piston of resin and the rear end portion of the output rod is normally contacted with the front surface of the rubber disc. When the brake booster is not operated, there exists a gap between the rear surface of the rubber disc and valve plunger. When the brake booster is operated, the rubber disc is not contacted with the valve plunger when the propulsive force of the output rod transmitted from the power piston through the rubber disc to the output rod reaches a predetermined value. Accordingly, the reaction force in compliance with the fluid pressure which is produced in a brake master cylinder is not added to the force acting on the valve plunger. When the propulsive force transmitted to the output rod reaches the predetermined value, the rubber disc is elastically transformed and protrudes into the small diameter opening portion of the power piston. As a result, the rubber disc will be contacted with the valve plunger and the reaction force will be added to the valve plunger.
In this prior art, however, the propulsive force of the output rod is set so as to be of a large value wherein the reaction force is not added to the valve plunger, and the volume of the rubber disc is elastically transformed and protrudes into the small diameter opening portion of the power piston increases, whereby the transformation of the rubber disc in the border portion between the small diameter opening portion and large diameter opening portion of the power piston become excessive and results in damage to the border portion of the rubber disc in use.